Can Short Term Indoor Air Quality Testing be Effective?

This post first appeared on the Green Building Advisor Website.

I own a CPS IAQPRO Smart Air professional indoor air quality monitor that I use on nearly every energy audit and building investigation I perform.  The tool measures particulate matter (PM2.5 and PM10), volatile organic compounds (VOC), carbon dioxide (CO2), temperature, relative humidity, pressure and dew point.  The question is, can short term air quality monitoring provide any useful feedback as to the indoor air quality of the home?  The answer, yes and no.

When starting an audit or investigation, I begin monitoring air quality shortly after entering the home.  Usually, my initial conversation with the homeowner starts at the dining room table.  Often, the kitchen or dining area is where the monitor ends up getting plugged in, simply because of the easily assessable outlets (the monitor is not battery operated).  The monitor is left running for as long as possible, typically at least an hour.  If a blower door test is planned, I need to record the monitor results before the test begins.  The blower door will flush air through the home, usually reducing any pollution levels.  The monitor ties to my phone, so I just take a screenshot of the app while it is recording.  The results shown (below) are pretty typical of what I see.  Before we get into a discussion about accuracy, let’s dive into what the numbers mean.

The PM2.5 and PM10 are the fine particulate matter that are floating in the air around us and are the most important metric an air quality monitor will record.  Every time we take a breath, some of these tiny particles ends up moving into our respiratory system.  The unit that particulate matter is measured in are micrograms per cubic meter.  The Environmental Protection Agency (EPA) recommends primary annual PM2.5 levels at 9.0 micrograms per cubic meter or less, this level was recently reduced from 12 micrograms per cubic meter.  There was also a reduction in the very unhealthy and hazardous limits.  You can read the EPA document on why the reductions were implemented here: 2024-pm-naaqs-final-overview-presentation.pdf (epa.gov)

The next air pollutant the monitor measures is carbon dioxide.  CO2 is measured in air concentrations of parts per million.  Carbon dioxide and water vapor are some of what is produced during the combustion process, burning fuels for energy.  Kind of funny that we as humans also produce carbon dioxide and water vapor when we exhale.  CO2 in extremely high rates is hazardous to our health.  We mostly use CO2 in indoor air as a proxy to our overall indoor air quality.  Outdoor air has CO2 levels of roughly 425 ppm, under 1,000 parts per million is considered a healthy level for indoor air.  OSHA has a weighted average limit value of 5,000 ppm for airborne exposure in any 8-hour work shift and a limit of 30,000 ppm for a 15-minute exposure limit.   CO2 limits on the International Space Station and in US Navy submarines are between 4,000 and 5,000 ppm.  ASHRAE has a good paper covering carbon dioxide, including a history of how we learned of C02 with regards to air quality.  pd_indoorcarbondioxide_2022.pdf (ashrae.org)

Volatile Organic Compounds (VOC) cover many different air pollutants including chemicals like benzene and formaldehyde.  Everything from new carpets and furniture to dirty dishes and body odors can increase VOC readings.  I’ve found that simply moving an air quality monitor from the kitchen to a bedroom can have drastic effects.  Though I take note of the VOC level, this reading is the least useful in the short-term air quality metric.  Under 500 ppb is the recommended level.

The other readings supplied by the CPS monitor are temperature and relative humidity along with the calculated dew point.  Though these metrics do not have a direct effect on air quality, high moisture content in the air can lead to the formation of mold and the temperature of different surfaces in the home may be below the dew point temperature, making them wet.  Most of my reports given to the homeowner have information on suggested indoor humidity levels.  The temperature, humidity, and dew point information provided by the monitor is useful for the audit or investigation.

Back to my original question, can short term air quality monitoring be helpful in an energy audit or building investigation?  As far as the accuracy of the readings is concerned, not really.  A one-hour monitor of indoor air is simply a very short snapshot in time.  To really understand air quality, monitoring should be conducted in weeks and months.  That being said, I still think the one-hour information can be useful.  Here is an example.  I was recently at a rental property performing an energy assessment where a young family, including a months old baby had moved into the recently renovated space.  The home was neat and clean.  When I checked the air quality monitor, I found an elevated level of particulate matter, both PM2.5 and PM10.

The air quality results are shown.  The first screen shot of the test was taken when the forced air furnace was off, the second, the furnace was in operation.  Both readings have concerning levels of particulate matter that we can assume is being caused by circulating the air through the ducted heating system.  The baby had recently developed some sort of respiratory illness, he had coughed several times during my visit to the home.  Was the home the cause?  I don’t know, but without this short-term test, no one would have known the home had elevated levels of particulate matter.

Another reason I feel short term air quality monitoring is important isn’t about the data that is presented to the homeowner, but rather the educational opportunity.  Most homeowners don’t have an awareness of what they may be breathing.  My hope in running an air quality monitor during an energy audit or assessment is to create some interest where the homeowner purchases their own monitor and begins long term testing of their home.  This testing may inform the owner that additional ventilation may be needed, or maybe it will show they consistently have good air quality.  We don’t really know without long term testing.

Personally, I own four different indoor air quality monitors (Awair, Airthings, Qingping, and the CPS) plus a pair of radon detectors (the Airthings has a built-in radon monitor plus a RadonEye dedicated radon monitor).  These air quality monitors have different price points ranging from around $150 to $300 (not including the CPS, which is closer to $600).  Do they all read the same when together?  No, they all have slight variations in all the readings, but what they all accurately show is trends.  This graph is from the Awair Element and is located in my bedroom.  The week of readings shows the ups and downs in carbon dioxide levels.  The dips are when the room was unoccupied, and the peaks are usually overnight hours.  The peaks and valleys withing the occupied times show when the forced air heating system operates.  (My significant other was ill for part of this graphed week; the Thursday reading is a more normal reading with an extended time of the room being unoccupied resulting in low CO2.

Similar graphs can be produced for all the air quality metrics.  Is the information useful?  I think so, overall, my home has decent indoor air quality.  Without monitoring, I would have no idea.

Indoor air quality monitors are improving, and prices are affordable for most.  I’m waiting for widespread use of monitors to control home ventilation equipment and rates of air exchange.  Until then, I’m going to continue to monitor my own home and try to convince others to do the same.